...
I usually disable it upon startup, and noticed that once disabled, you can't re-enable it while the car is running...

Not true. While I've coded some things on my car myself, the ASS recoding was done by BMW at the VPC. I can turn it off and turn it on at will, even while the car is running.

If ASS is on and the engine is stopped, turning it off starts the engine. If ASS was off and the car is in motion, turning it on will most likely cause the engine to stop at the next full stop (depending on the normal ASS variables, like temperature, steering angle, position of the gear selector for AT).